This invention relates generally to an engine-driven power generating system, and more particularly to an engine-driven power generating system for rectifying the a-c output generated by an a-c generator driven by an engine into d-c power that is in turn converted into a predetermined level of low-frequency a-c voltage using an inverter circuit driven by a pulse-width modulated (PWM) signal; and having an emergency relay circuit device for protecting the engine-driven power generating system against abnormal operating condition.